This invention relates generally to a heat and pressure fuser for an electrophotographic printing machine, and more particularly the invention is directed to a dual tamp arrangement for elevating the surface temperature of a fuser roll and means for controlling the operation of the lamps for optimizing the operation of the fuser for fusing different size substrates.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to selectively dissipate the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an adhesive material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon. In these fusing systems, however, since the toner image is tackified by heat, it frequently happens that a part of the image carried on the supporting substrate will be retained by the heated fuser roller (commonly known as hot offset) and not penetrate into the substrate surface. The tackified toner may stick to the surface of the fuser roll and offset to a subsequent sheet of support substrate or offset to the pressure roll when there is no sheet passing through a fuser nip resulting in contamination of the pressure roll with subsequent offset of toner from the pressure roll to the image substrate. The foregoing problem is exacerbated when this type of fuser is required to fuse different size substrates.
The following publications may be relevant to the invention:
U.S. Pat. No. 4,434,353 granted to Marsh et al on Feb. 28, 1984 discloses an apparatus which includes a plurality of heating elements to fuse a powder energized in an ordered sequence corresponding to the surface area of the powder image being fused.
U.S. Pat. No. 5,227,270 granted on Jul. 13, 1993 to Scheuer et al discloses a single pass tri-level imaging apparatus wherein a pair of Electrostatic Voltmeters (ESV) are utilized to monitor various control patch voltages to allow for feedback control of Infra-Red Densitometer (IRD) readings.
U.S. Pat. No. 3,832,524 granted to Takiguchi on Aug. 27, 1974 discloses a copying machine having a heating unit including a central heating element and two end heating elements. A switch connects the central and two end heating elements in parallel across a voltage source when the larger of two different width copy sheets is used. The switch disconnects the two end heating elements from the voltage source when the smaller of two different width copy sheets is used.
U.S. Pat. No. 4,075,455 granted to Kitamura et al on Feb. 21, 1978 discloses a fusing device having a plate and a heater disposed transversely of the plate. The heater is divided into heater sections in the direction of the width of the copy sheet. A temperature sensor controls the power to each of the heater sections. The power furnished to the heating elements depends upon the width of the copy sheet.
U.S. Pat. No. 4,928,147 granted to Baumann et al on May 22, 1990 discloses a device wherein image defining toner is transferred to cut sheets and fused at one of two fuser stations. One station is positioned to fuse the image on one side when duplex copying or printing is selected. The other fuser station performs fusing when simplex copying is selected but fuses the second side copy for duplexing. When flash lamps are employed for the fuser station, the lamps are composed of multiple bays with a single power source coupled to each bay in sequence so that the power source size need only accommodate the power level demand of one bay. The time between trigger pulses is extended by commencing fusing with an intermediate bay followed by the initial bay and then the final bay as the image area requiring fusing passes along its path in proximity to the faces of the flash lamp bays.
U.S. Pat. No. 4,531,824 granted to Benzion Landa on Jul. 30, 1985 discloses an improved heater for electrophotographic copiers wherein a plurality of laterally spaced heaters dry or fix the transferred image along narrow longitudinally extending strips to prevent smearing of the image from subsequent contact with feed wheels or guides.
U.S. Pat. No. 4,474,456 granted to Kobayashi et al on Oct. 2, 1984 disclosed a heat-fixing device including a heat plate along which a transfer medium bearing thereon a toner image is advanced during which the toner image becomes fixed to the transfer medium by fusing, the heat plate including a plurality of heating sections arranged along the direction of advancement of said transfer medium each of the heating sections being provided with at least one electrical heater. The device also includes a control circuit for controlling the operation of the heaters such that, under a non-steady-condition, only selected ones of the heaters are set in operation thereby only the corresponding heating sections are positively heated; whereas, under a steady-state condition, all of the heaters are operated at the same time thereby maintaining all of the heating sections at a predetermined temperature.
U.S. Pat No. 4,825,242 granted to Michael R. Elter on Apr. 25, 1989 discloses an apparatus in which an image is fused to a sheet during a copy run. The number of sheets having images fused thereto is counted during the copy run. Heat is applied to at least the images of successive sheets of the copy run. The heat being applied to the images on successive sheets is controlled in response to the number of sheets counted. Two fuser lamps are energized in accordance with the size of substrate being fused.
U.S. Pat. No. 5,206,694 granted to Ohira et al on Apr. 27, 1993 discloses an image forming apparatus comprises an image forming portion for forming toner images on a sheet of paper, a fixing portion for fixing the toner images on the sheet of paper by heat, and a counter for counting the number of sheets on which images are fixed from the time of turning on of the power. The image forming portion forms toner images on the paper at a low speed until the counter counts a prescribed number. Since the number of sheets to be fixed is reduced at the start of operation, the temperature of the fixing portion is not very much lowered. Namely, the fixing operation can be started when the temperature of the fixing portion reaches a prescribed value. Therefore, the time required for warming up the image forming forming apparatus can be reduced.
U.S. Pat. No. 4,585,325 granted to Euler on Apr. 29, 1985 describes a heated fixing roller having two heating elements located inside the roller. The heating elements are connected to a control system and a sensor to control the current being supplied to the heating elements. There is disposed within the heating roller a second electrical heating element, which is connected to the control system through an additional temperature sensor. This second element runs parallel to the roller axis. The first heating element has a middle coil in the middle zone of the heating roller and the second or other heating element has two end coils disposed near the edge zones of the heating roller adjacent the left and right of the middle zone. The middle coil in the middle zone overlaps with each end coil in each edge zone, and the current supply to the heating elements is controlled so that on the surface of the heating roller there results a surface temperature distribution in the axial direction which exhibits a local minimum or maximum in the middle zone and maxima near the two end faces of the roller, one of the two temperature sensors is located near the extremum of the surface temperature in the middle of the roller, and the other temperature sensor is located near the middle of one of the edge zones of the heating roller.
U.S. Pat. No. 5,041,718 granted to k'Hondt et al on Aug. 20, 1991 discloses a device for fixing a powder image on a receiving support using heat that consists of an image transfer roller internally provided with a first heating element having the same heat-generating power over the entire length of the image transfer roller, and a second heating element which has a higher heat-generating power in the edge zones of the image transfer roller than in the middle zone of the roller, and a pressure roller internally provided with a third heating element which like the second heating element has a higher heat-generating power in the edge zones than in the middle zone. The device may be in a warm-up condition in which the temperature of the rollers is not yet at the working level, stand-by condition in which the temperature is at the working level but in which no fixing is carried out, and a fixing condition in which fixing is carried out. During warm-up, all the heating elements generate the maximum power. During stand-by, the first heating element is switched off and the effective powers of the second and third heating elements are set to a much lower value, so that the ratio between the amount of heat generated in the edge zones and the amount of heat generated in the middle zone is greater than during warm-up. During fixing, the effective powers of all three heating elements are set to a higher value than during stand-by, but the ratio between the amount of heat generated in the edge zones and the amount of heat generated in the middle zone is lower than during stand-by.
An imaging device or the type disclosed in the '270 patent requires the fuser to fix two different toners (black and color) simultaneously. Graininess in black solid areas resulting from interactions of the black latent image with the color developer brush is minimized by running with a high transferred mass. The high mass requires higher surface temperature to meet the desired fix performance which reduces the difference between nominal temperature and the onset of hot offset. Finally, the increased rubber coating thickness (needed to meet fuser roll life targets) increases the lag time of the system which, in turn, increases the steady state temperature fluctuations resulting from the simple on-off controller. Since the existing fusers are not well suited for fusing the types of images contemplated there is need for an improved fuser for such purpose.